Instagram is currently testing a comments feature on the platform by allowing users to comment on an image inside a multi-photo post. Some are entitled to hyperlink their comment into the respective photo or video using a number assigned on the visual.
The option will come in handy since users shall @mention an image's number within the carousel then append their comments to it.

For example, if you wanted to comment on the second picture of the display, then you will write "@2" followed by your comment.
Perhaps it is just another way that will actually drive more focused engagement and encourage more interaction around every element, while clearing the confusion when commenting on a carousel post.

It would appear that carousel posts are a new focus for the company as it aims to get people to share more personally within the app. Last October, the app began testing collaborative carousels, which allowed others to contribute to the updating of the carousel feed while more recently, the firm also began testing expanded carousels, up to 20 images available in one update.

With both options, there might be value in having an easy way to directly comment on each frame in a carousel, and enable more engagement and interaction inside the main IG feed.

Instagram is testing the new option to tag carousels with a limited set of users at this stage.